1. Technical Field
The inventive arrangements relate generally to mobile telecommunications systems and more particularly to a more efficient use of RF spectrum allocation for repeater/base station backhaul links.
2. Description of the Related Art
In a wireless telecommunication system, a base station communicates with mobile communication devices via communication channels, known in the art as ground links. By itself, a base station can only cover a limited area with ground links. This limited coverage area is referred to in the art as a cell. Other devices, such as repeaters, are sometimes used to expand the range of a base station to cover a larger geographic area. To a limited extent, non-translating repeaters extend the range that a base station can cover within the same cell. A frequency translating repeater can provide coverage within the same cell or for a separate cell from the cell of its serving base station. Repeaters are typically placed beyond the range of a base station's ground link so as to expand the base station's service to cover those cells. A group of cells covered by a base station utilizing repeaters is known a cell cluster. The present invention applies generally to the class of repeaters known as frequency translating repeaters.
A backhaul communication link allows the base station to communicate with the repeaters for receiving and transmitting information to/from the mobile communications devices. Some base station units and repeaters are configured with a backhaul channel that operate “in band”, i.e. in a portion of the limited frequency spectrum allocated for ground links. Other base stations may support the backhaul in a licensed or unlicensed band other than the band allocated for ground links. However, use of in unlicensed bands is typically undesirable due to interference from uncontrolled sources. Likewise availability of spectrum in a licensed band other than the band used for ground links may be limited as well as costly. In conventional systems, a different backhaul carrier frequency is required for communication between a base station and each associated frequency translating repeater. However, in systems that select the backhaul from a portion of the limited frequency spectrum allocated for ground links, such multiple backhaul frequencies reduce the available ground link bandwidth.
A base station may support a number of repeaters that are geographically distributed in any direction from the base station. The repeater would typically utilize a narrow beam antenna to receive downlink signals and transmit uplink signals to its serving base station. This narrow beam antenna focuses transmitted uplink signal to its serving base station and also limits interference to unintended base stations. However, the base station typically would employ an omni-directional or broad beam directional antenna that could support multiple backhauls to different geographically diverse repeaters. Because the downlink (base station-to-repeater) backhaul is transmitted in a broad direction rather than focused to the intended repeater, this broad transmission generates interference for unintended repeaters thus limiting spectral efficiency. Likewise, more power is required to be transmitted than would be necessary than if the base station antenna were using narrow beamwidth antenna. On the uplink the base station may receive a signal from an unintended repeater on the same frequency as an intended repeater.
Smart antenna systems are known in the art as a way for multiple communication links to improve spectral efficiency by spatially isolating those communications links from each other thus reducing interference. For example, switched beam systems are available that may consist of a plurality of narrow antenna beams arranged in a pattern to cover an omni-directional area. In switched beam systems, each antenna requires one or more dedicated transceivers and transmit amplifiers together with associated RF cabling. This arrangement permits communications between the base station and a plurality of remote transceivers to occur concurrently on the same frequency, but through different antennas.
Another smart antenna system is comprised of an array of antenna elements that are used to perform adaptive spatial processing. The system works by electronically forming RF beams and nulls by adjusting the phase and amplitude of each communication channel through each of the antennas in the array. Adaptive spatial signal processing applied to RF signals of each of the antenna elements permits RF energy to be focused to/from a specific direction to/from the same base station, thus reducing interference between remote communication devices on the same frequency that are spatially separated.
Smart antennas have been applied to support spatial division multiple access (SDMA) systems for improving spectral efficiency. These systems make use of the spatial separation of remote communication devices enabling multiple remote devices to communicate with an SDMA base station on the same frequency. SDMA can be implemented using adaptive antenna systems or switched beam systems.
Much study has been performed to apply SDMA to mobile networks; however, thus far in practice, SDMA systems have been limited to fixed wireless networks, such as wireless local loop (WLL) systems, due to the tremendous computation power required to track a large number of mobiles and monitoring spatial separation of those on the same frequency to prevent unacceptable degradation in signal quality.
Smart antennas have been applied to improve spectral efficiency by reduction of interference allowing higher frequency reuse across a mobile network. Each cell in a cell cluster may use unique RF frequencies for that cell; however, those RF frequencies may be reused in cells of another cell cluster. By reducing interference through the use of smart antennas, cells of different clusters using the same frequencies may be placed geographically closer, allowing those frequencies to be used more often, thus improving spectral efficiency.
Hence, in a network making use of frequency translating repeaters, what is needed is a mobile communication system that can take advantage of existing smart antenna technology for more efficient use of frequency spectrum, in particular the loss of spectral efficiency due to the use of backhaul communication channels.